This invention relates to an improved process for the removal of sulfur dioxide from flue gas containing sulfur dioxide and other components, including oxygen.
In general, pollution control regulations now restrict severely the volume of sulfur compounds which may be discharged from furnace stacks. Because the flue gas from furnaces is generally the product of complete combustion in the presence of excess oxygen, the flue gas has an oxidative character, and the sulfur is present chiefly as sulfur dioxide. Various methods for the removal of sulfur dioxide from these streams have been proposed.
In one such method, sulfur dioxide is removed from the flue gas at elevated temperatures in the presence of a copper-containing solid acceptor. The copper acceptor binds the sulfur dioxide under oxidative conditions, with the formation of copper sulfate. One advantage of this process is that sulfur dioxide may be regenerated without addition of heat, by use of a reducing atmosphere. The acceptor is then again suitable for acceptance of sulfur dioxide. During regeneration, a regeneration off-gas becomes available which contains sulfur dioxide in much higher concentration than that of the flue gas. This sulfur dioxide may be recovered by known procedures. In the regenerated acceptor, the copper is in metallic and/or oxidic form. Any metallic copper present is re-oxidized in a subsequent acceptance period.
In the practice of this flue gas desulfurization process, use is made of reactors having separate inlet and outlet ducts for the flue gas and the reducing gas. These ducts or lines are provided with valves near the reactors for regulating the flow of the reactants. During the regeneration of the loaded acceptor, the valves in the inlet and outlet ducts for the reducing gas of the relevant reactor are open, and the valves in the inlet and outlet ducts for flue gas of that reactor are closed.
Because of the large volume of gas being treated, the relevant flue gas ducts will have a considerable diameter. Accordingly, valves having a large diameter must be used. In these valves, some leakage may occur locally.
During the regeneration of a loaded acceptor, the reducing gas flows on the reactor side of the valves in the flue gas inlet and outlet ducts. Flue gas leaking into the duct will not pose a problem, since it will immediately be removed. However, since the flue gas is stationary on the non-reactor side of the closed valves in the flue gas inlet and outlet ducts, any leak into the dead ends of these ducts may be sufficient to form an explosive mixture.
Although the flue gas ducts could be purged with steam or an inert gas, this method has several drawbacks, since special facilities are required therefor, such as a boiler and a separate duct system with valves. Moreover, the capacity of the flue gas desulfurization apparatus would be decreased by the steam or inert gas passing through the reactor when opened for flue gas.
Accordingly, the invention aims at eliminating this hazard and at providing economically attractive means to prevent the formation of explosive gas mixtures in the flue gas ducts.